1. Field of the Invention
The invention relates to a printing apparatus, more specifically, to a printing apparatus generating interpolation data which interpolates dot data to obtain a high-quality printing output.
2. Description of Related Art
In laser printers, interpolation dot data generated from dot data is output for the purpose of high dot density and high-level gradation to produce a higher-quality printing output. The interpolation dot data has a higher dot density than the dot data, that is, a dot of the interpolation dot data is smaller than that of the dot data for interpolating the jagged outline of the characters to be output.
In a laser printer capable of carrying out both a printing operation with low resolution (300 dpi) and a printing operation with high resolution (600 dpi) alternatively, the common interpolation dot generating means, for both printing operations with low resolution and the high resolution, comprises one chip of LSI which is used to generate interpolation dot data for both the low resolution and high resolution printing.
However, a nice appearing and accurate printing output for both resolutions cannot be obtained because the common interpolation dot generating means is used to generate the interpolation dot data for both the low resolution printing and the high resolution printing in the above-described laser printer.
Usually, toner particles and the photosensitive drum intended for printing at 300 dpi have been used for printing both 300 dpi and 600 dpi and the interpolation dot generating means of 300 dpi has been most commonly used when both interpolation dot data for 300 dpi and for 600 dpi are generated. As a result of using the common interpolation dot generating means, the problems described below are encountered.
In printing with the low resolution of 300 dpi, interpolation data X is generated by the interpolation dot generating means to interpolate the jagged outline of the characters to be output. Even if the jagged outline is interpolated appropriately by the interpolation data X, there occurs a problem that a nice appearing printing output cannot be obtained when printing in the high resolution of 600 dpi using the interpolation data X.
That is, in the above-described printer used with toner particles and the photosensitive drum intended for printing at 300 dpi, when such toner particles, for 300 dpi, are used in printing with the high resolution of 600 dpi, it becomes difficult to print dots of a high density because the particle diameter of the toner particles for 300 dpi is larger than desirable for printing with 600 dpi. Further, the photosensitive drum intended for the low resolution of 300 dpi may not accept the irradiation light for printing dots of high density, that is the photosensitive drum for 300 dpi may not be sensitive enough to the irradiation light for printing at 600 dpi. Therefore, the photosensitive drum is not charged precisely and accurately and the quality of the printing output cannot be appropriately improved as would be expected with this increased dot density.
In addition, if the interpolation data generating means for 300 dpi generates the interpolation data for 600 dpi, there are problems as described below and the quality of a printing output is not appropriately improved.
As shown in FIG. 5, dot data 40 of 4 dots in the vertical direction and 8 dots in the horizontal direction is generated at the low resolution of 300 dpi. The interpolation data 42 of the dot data 40 is generated by the interpolation data generating means for 300 dpi.
As shown in FIG. 6A, dot data 46 of 4 dots in the vertical direction and 8 dots in the horizontal direction is generated at the high resolution of 600 dpi in the same way as in the case of 300 dpi. The interpolation data 48 of the dot data 46 is generated by the interpolation data generating means of 300 dpi in the same way that the interpolation data 42 of the dot data of 300 dpi is generated. However, if the interpolation data 48 is generated by the interpolation data generating means in the same way that the interpolation data 42 is generated, extra interpolation data might be generated or necessary interpolation data may not be generated.
Therefore, there has been a problem that a printing output is not improved appropriately. For example, as shown in FIG. 6B, the interpolation dot data 48E generated as the interpolation dot data 48 of the dot data 46 of 600 dpi is an extra interpolation dot data that is not needed. On the other hand, in addition to the interpolation dot data 48 of 600 dpi generated by the interpolation dot data generating means of 300 dpi, the interpolation dot data 48N is necessary to obtain a beautiful printing output. However, the interpolation dot data 48N is not generated by the interpolation dot data generating means of 300 dpi. Therefore, there has been a problem that the quality of the printing output is not improved appropriately.
Further, if the above-described printer has interpolation dot data generating means of 600 dpi which is used for generating both interpolation dot data of 600 dpi and 300 dpi instead of the interpolation dot data generating means of 300 dpi, the interpolation dot data of 600 dpi is capable of improving the jagged outline of the character of 600 dpi but when it is used for printing at the low resolution of 300 dpi, there are problems noted as follows. Similar to the problems with the interpolation dot data generating means of 300 dpi, in some instances extra dot data will be generated and printed and in other instances necessary dot data will not be generated and printed so that a nice and accurate image cannot be obtained. For example, the character becomes thick or an edge of the character is rounded off.